Concerning the polymerization of propylene, Ziegler-Natta catalysts can be made form or containing a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound. When higher crystallinity of the polymer is desired, an external donor can be used to obtain higher isotacticity. Esters of phthalic acid can be used as internal donors in catalyst preparations. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor.
The use of this catalyst system has raised health concerns with the use of phthalates.